It's a balance between the time needed to charge the battery and longevity of it's life. Besides that, depending on the current capacity and the level of discharge, different charging algorithm might be used.

AFAIK so called "rapid chargers" (those for standard batteries, e.g. AA, AAA) are using pulses of high current in some particular pattern to charge faster. I don't have any factual information on how that affects battery lifespan, but I'm pretty sure it comes with a price.

Same happens with N900 - charging from a computer (by connecting it to an USB port) is noticeably slower then charging from the "dedicated charger". The charging "machinery" there detects the type of the power source and adjusts algorithm.

Again, it's a question of balance.

I haven't checked if it charges faster with a dedicated "charger" comparing to charging from PC's USB port. It might be faster, as more current is available (same applies to special high current USB ports on some USB hubs/notebooks, which provide 1.2 A of current).

BTW, having an AMOLED display on board, device can consume quite a bit of the current provided by the power supply while charging. So, it should be possible to make charging a bit slower, if display will be on and showing some bright image (the brighter is the image, the more power display drains). Adding CPU load to that counts too.